Polyethylene resins having bimodal characteristics include resins that comprise two components having different properties, such as for instance two components of different molecular weight, i.e. a component with a relatively higher molecular weight component (HMW) and a component with a lower molecular weight (LMW) component; two components of different densities; and/or two components having different productivities or reaction rates with respect to co-monomer.
The use of metallocene catalysts in the polymerization process or copolymerization process of ethylene is a relatively recent development. Processes for producing bimodal polyolefins in general and bimodal polyethylene in particular in the presence of metallocene catalysts have been described.
Bimodal polyethylene resins can be prepared according to different methods. Bimodal polyethylene products can for instance be made by physically blending different monomodal polyolefin products which are independently produced. However, a problem with those physically produced bimodal products is that they usually contain high levels of gels.
Alternatively, bimodal polyethylene can also be prepared by sequential polymerization in two separate reactors that are serially interconnected. In such sequential polymerization process in one reactor, one of the two components of the bimodal blend is produced under a set of conditions maintained in the first reactor, and transferred to a second reactor, where under a set of conditions different from those in the first reactor, the second component is produced having properties (e.g. molecular weight, density, etc.) different from the first component.
However, using metallocene-based catalyst systems to catalyze the preparation of bimodal polyethylene in serially connected reactors, results in polymer fractions that may be difficult to mix with one another. A problem associated with known bimodal polyethylene products is that if the individual polyethylene components are too different in molecular weight and density, they may not be homogeneously mixed with each other as desired. As a consequence harsh extrusion conditions or repeated extrusions are sometimes necessary which might lead to partial degradation of the final product and/or additional cost. Thus the optimum mechanical and processing properties are not achieved in the final polyolefin product. Also, bimodal polymer particles produced may not be sufficiently uniform in size, and hence segregation of polymer during storage and transfer can produce non-homogeneous products.
Another technique for preparing bimodal polyethylene consists of preparing bimodal polyethylene resins in a single reactor. Production of polyethylene with a bimodal molecular weight distribution (MWD) in a single reactor has long been a goal of the polyolefin industry because single reactor configurations are significantly cheaper to build, have improved operability, and enable quicker product transitions than multi-reactor configurations. A single reactor can also be used to produce a broader range of products than a set of cascaded reactors can do.
Bimodal polyethylene resins can be prepared in a single reactor by employing two distinct and separate catalysts in a same reactor each producing a polyethylene component having certain properties. In an example, bimodal polyethylene can be produced by combining two different single site catalysts in a single reactor, as is described for instance in WO 2006/045738.
In another example, WO 95/11264 discloses a process for the preparation of polyethylene blends comprising a high molecular weight component and low molecular weight component. The catalyst system used in this process contains two different transition metals, one of which is a metallocene, and one of which is a non-metallocene. The resulting blends embrace a broad spectrum of product compositions, determined by the weight fractions and molecular weights of the individual components.
Alternatively, a single dual site catalyst system can be used to produce bimodal polyethylene in a single reactor, as is described for instance in WO 2004/029101.
However, a problem of bimodal polyethylene preparation in a single reactor, is that the catalysis reaction may be difficult to control, and that highly sophisticated catalytic systems are required.
In view of the above, there remains a need in the art to provide an improved method for preparing a bimodal polyethylene product with improved control and convenience in a single reactor. It is in particular desirable to find ways of making homogeneous bimodal polyethylene in a single reactor having desired and controllable properties.